Physics AS electricity

revision of physics as electricity.

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  • Created on: 03-04-11 10:59

Current and Charge

Current - a flow of electrons

each electron carries a charge of 1.6 x 10^-19.

Current is rate of flow of charge therfore-

Q=IT, charge = current x time

The simplest current is in a cathode ray tube, in a vacuumn, the cathode is heated releasing electrons towards the anode. It must be in a vacuumn to avoid collisions. Typically 5 x 10^14 pass a point in one second.

One coulomb is the charge supplied by a current of one amp in one second.

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current in a wire

Which metal ?

Copper for a connecting wire.

Copper or aluminium for a motor.

Tungsten for alightbulb filament.

Nickel- Crome alloy for a heating element.

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Convential current flows from positive to negative, however, electrons really flow from negative to positive.

Ameters measure current-

Analogue ammeters rely on a small coil rotating in a magnetic field. Digital ammeters use a current sensor.

Ideally the reistance of an ammeter is zero so it does not affect the current it is measuring however in reality the resistance is very low.

Ammeters must be placed in series.

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Kirchoff's first law.

The sum of the current entering any junction is always equal to the sum of the current leaving the junction.

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Electron drift velocity

How fast electrons (not current) move.

Collisons of electrons with lattice atoms means their dirft velocity is greatly reduced (e.g. in copper). Factors affecting drift velocity - density and atomic mass.

Equation -

I = nAve

I = current, n = number density, A - cross sectional area of the wire,

v = mean drift velocity, e = elctronic charge (1.6 x 10^-19)

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Drift velocity in other materials

In conductors, there are large numbers of free elctrons so number density is high so current will be high.

In insulators there are low numbers of free elctrons so number density is low so current is low.

semiconductors have very low number density compared to conductors, however  it can be increased by adding an impurity. The low number density means that drift velocity is much higher than in good conductors.

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Electro motive force (emf)

This is the energy transferred per unit charge when one other type of energy is converted into electrical energy.

Emf = energy transferred / charge

the unit is volt.

 1 volt = 1 joule per coulomb

Typical values = 1.5 volts in a torch cell, 6 volts in a battery and 230 volts from the mains.

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Potential difference

This is the electrical energy transferred per unit charge when one other energy is converted into electrical energy.

Emf refers to electrical supply e.g. a battery, where as p.d refers to a component of a circuit. The unit is still a volt.

The voltmeter must be in parallel in a circuit and the resistance should be infinite but is in reality very high.

Power = current x potential difference.

remember than when p.d = 1 volt and charge= 1.6 x 10^-19. e x v = one electron Volt = W.

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Ohm's law

The resistance of a component is proportional to the p.d. across it if conditions remain constant.

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I-V characteristics - ohm's law does not apply as when the p.d. increases the temperature increases so the resistance in the filament increases.

if a lightbulb is made more efficient its lifetime will be decreased, so a compromise if made between lifetime and efficiency. Some modern lightbulbs are much more efficient but -

  • some contain harmful gas
  • they take a long time to reach full intensity.
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Light Emitting diodes.

These emit visible light when a current passes through them. They have many advantages

  • they switch on imediately
  • they have a long lifetime
  • very robust
  • very reliable
  • very versitile
  • work with low p.ds

But they are diodes and so have to be the right way round.

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The property of a material.

If resisivity is high the material is a poor conductor.

resisivity is proportional to temperture becuase - ions have more kinetic energy so electrons have to move through more turbulent atoms.

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Electrical power

Power is the rate at which energy is transferred.

P = IV

P = I^2 X R

P = V^2 / t

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Series circuits

Rtotal =R1 +R2 +......

Batteries have internal resistance which is usually small and constant.

E = I(R + r ) or E = V + Ir

The voltage you get out of the cell after internal resistance is called termal p.d.


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Kirchoff's second law

In any closed loop the sum of the emfs equals the sum of the p.d.s.

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In a paralell circuit

I = I1 + I2 + I 3 ....

I = E/R1 + E/R2 + E/R3 ....

I = E/R

E/R = E/R1 + E/R2 + E/R3 ...

dividing by E

1/R = 1/R1 + 1/R2 + 1/R3 ....

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